Silicate mortars and cements are known for almost a century as useful materials for construction of brickwork and monolithic linings resistant to all acids except hydrofluoric acid. Originally, they consisted only of mixtures of sodium water glass solutions and suitable fillers such as sand, gravel, clay or other minerals. Curing was dependent on the carbon dioxide content in the air and was very slow, taking weeks and months. A major improvement was the addition of acid releasing hardening compounds to the mortars, making it possible to obtain a cure in days instead of weeks. A variety of inorganic and organic hardeners were tried and some are still used. The most useful ones proved to be the fluosilicates, expecially the sodium and potassium fluosilicates.
In later years it was found the potassium water glass mortars, although higher in price, had certain advantages over sodium water glass mortars, especially in sulfuric acid service. Sodium compounds may form salts with a high amount of crystal water which results in the destruction of the mortar. Potassium silicates are much less prone to the formation of these so called "growth salts" and are more resistant to acids when utilized for chemical resistant masonry construction.
The need for corrosion resistant refractory linings for incinerators and chimneys discharging gases from sulfur bearing fuels which may condense to form sulfuric acid is becoming more important every day. Chemical resistant brick linings have been used extensively for many years in this high-temperature acid atmosphere. However, the difficulty of lining and/or repairing irregular contours and overhead surfaces with brick and the costs associated with these difficulties has led to the development of acid resistant cements which can be spray applied pneumatically to form monolithic linings. Gunned monolithic linings have the advantage that curved or irregular surfaces can be covered uniformly, the linings bond well to steel or existing linings, the linings are dense, homogenous and with low permeability. Monolithic linings can be applied in any desired thickness greater than 1/2" to horizontal, sloping, vertical and overhead surfaces. Thicknesses greater than 1" can be applied with suitable anchorage systems. In addition, gunned linings have the advantage of lower labor installation costs, ease and speed of application and they are invaluable in making repairs to in-service equipment.
One of the early developments in the gunning of acid resistant mortars was the application of wet gun cements. In the wet gun application the cement is premixed with water and is pumped through a hose to a nozzle fitted with a compressed air source.
The wet guniting method of application is not conducive to the installation of acid proof monolithic products because of the low density of the installed material with lower strength product. Also, the excessive amount of water required for pumping the cement leads to a reduction in desirable physical properties of the finished cement and the frequent blockages in the supply hoses causes many application difficulties.
The most ideal application method is with the dry gun. Here the dry cement powder is delivered at high pressure to the nozzle where the liquid binder is sprayed into the powder at the point of discharge and is impacted on the surface. This produces the optimum high density lining required for corrosion resistant service.
In the field of acid resistant brick linings one of the major developments was the introduction of sodium silicate component cements. The cement disclosed in U.S. Pat. No. 3,813,253 is typical of the single component sodium silicate cement mortar. By single component cement is meant a cement powder which requires nothing further than the addition of water to produce the desired cement. Attempts to gun the single component sodium silicate cements have been less than satisfactory. The defect of growth salts noted above in connection with sodium silicate mortars is also present in sodium silicate gunned cements.
A further disadvantage found in the application of monolithic linings utilizing a single component 100% sodium silicate based cement is the slow wetting out action of the dry sodium silicate. The phenomena results in the gunited product sloughing off the substrate when small amounts of excess water are added at the nozzle during placing of the cement. On the other hand, if too little water is added then the desired chemical reactions cannot go to completion resulting in an inferior quality cured product. This narrow range of acceptable water addition is very critical and is the complete responsibility of the nozzle operator.
As a consequence of the slow wetting out of sodium silicate cements the nozzle operator can only place 1/2 to 2/3" thicknesses of cement on vertical or overhead surfaces because the weight of thicker linings pull the cement off of the surface before a set can take place. To overcome this application problem but at the expense of optimum chemical resistance, formulators have added a percentage of hydraulic setting cement binders such as Portland Cement and calcium aluminate cement, which are not acid resistant materials.